It is known to employ a monitoring unit for the purpose of monitoring current-consuming loads in respect of their electricity consumption in order thereby to deduce from interruptions of the load current or generally in the event of predefined threshold values being undershot or exceeded that a malfunction has occurred among the loads. Thus, for example, it is usual in the case of motorized vehicles to use a central control device (also known as a body control unit (BCU)) for controlling various lights of the motorized vehicle, front lights and tail lights for example, and in the process for establishing on the basis of the electricity consumption, i.e. of the load current, whether the lights are functioning properly or not. For this purpose a lower threshold is defined for the load current.
More recently there has been an increasing trend toward replacing the incandescent or halogen lamps etc. used hitherto for lights of said type by alternative lighting means, such as in particular LEDs (light-emitting diodes). This reduces electricity consumption and correspondingly, in the case of current monitoring for the purpose of fault detection, the load current thresholds for fault detection are set lower.
The lamps, in particular also LEDs, are typically controlled by means of direct current converters, referred to hereinbelow as DC/DC converters for short. A DC/DC converter of said kind is mostly accommodated in a separate control module that is arranged between the central control device, the BCU, and the loads, i.e. LEDs. During operation, (higher-frequency) electromagnetic interference, caused, for instance, by the DC/DC converter, can be produced in this area and, if no other measures are taken, can reach the vehicle electrical system. The proper operation of other control devices can be compromised by said sources of interference, and interference signals can also be emitted into the environment via cables acting as antennas. Filter devices of the kind to which the invention relates are used in order to prevent said interference as effectively as possible. It can nonetheless happen that the current to the filter device or, as the case may be, to the load becomes discontinuous without a malfunction of the load being present, as a result of which it becomes difficult for the central control device, the BCU, to differentiate the case of proper operation from a fault situation.
A discontinuous current can be attributable in particular to the following causes:    1. the DC/DC converter “misses”, which is to say that the current is not consumed continuously, constantly returning to 0 under certain conditions.    2. the filter elements used at the input in the filter device store energy and feed current back into the onboard electrical system of the motorized vehicle when fluctuations occur in said system.
In both cases the control device (the BCU) could diagnose a line break.
In order to solve the problem of the discontinuously operating DC/DC converter, a suitable topology can simply be chosen and at the same time it can be ensured that the input current remains essentially constant.